1. Field of the invention
The present invention relates to a nonvolatile semiconductor memory, and more specifically to a nonvolatile semiconductor memory configured to store information by utilizing polarization of a ferroelectric material.
2. Description of Related Art
A nonvolatile semiconductor memory utilizing a ferroelectric material as a medium for storing information, has an advantage that if deterioration of a ferroelectric material film does not occur, information can be stored for a long term of time, and further, is expected that it can make it possible to reduce a memory cell size and therefore to realize a memory having a large storage capacity.
Reza Moazzami et al, xe2x80x9cA Ferroelectric DRAM Cell for High-Density NVRAM""sxe2x80x9d, IEEE ELECTRON DEVICE LETTERS, Vol. 11, No.10, October 1990, Pages 454-456, (the disclosure of which is incorporated by reference in its entirety into this application) proposed one example of the above mentioned conventional nonvolatile semiconductor memory, in which a capacitor dielectric of a DRAM memory cell capacitor is formed of lead zirconate titanate (PbZr1xe2x88x92xTixO3)
Referring to FIG. 1, there is shown a diagrammatic sectional view of the nonvolatile semiconductor memory proposed by Reza Moazzami et al. On a principal surface of a P-type silicon substrate 101, a device isolation oxide film (field oxide) 102 is formed by a selective oxidation such as a LOCOS (local oxidation of silicon) process, and within an active region confined by the device isolation oxide film, a gate electrode 103 is formed through a gate insulator film on the surface of the substrate. A source region 104 and a drain region 105 are formed in a surface region of the substrate at opposite sides of the gate electrode 103, so as to locate the gate electrode between the source region and the drain region. A first interlayer insulator film 106 is formed to cover a whole surface of the substrate, and a Pt film 107 is formed on the first interlayer insulator film 106, above a position of the gate electrode 103. Furthermore, a PZT (PbZr1xe2x88x92xTixO3) film 108 is formed to cover the Pt film 107. A second interlayer insulator film 109 is formed to cover a whole surface of the substrate including the PZT film 108. In addition, contact holes are formed to reach the drain region 105 and the PZT film 108, respectively, and an aluminum wiring conductor 110 is formed on the second interlayer insulator film 109 to contact with the drain region 105 and the PZT film 108 through the contact holes.
Referring to FIG. 2, there is shown a diagrammatic section view illustrating another example of a conventional nonvolatile semiconductor memory utilizing a ferroelectric material film, in which a gate insulator film of a transistor is formed of a ferroelectric material film.
As shown in FIG. 2, a device isolation oxide film 2 is formed on a principal surface of a P-type silicon substrate 1, and a ferroelectric material film 4C is formed on the principal surface of the substrate 1 to constitute a gate insulator film. A gate electrode 5A is formed on the ferroelectric material film 4C, and a source region 7 and a drain region 8 are formed in a surface region of the substrate at opposite sides of the gate electrode 5A, so as to locate the gate electrode between the source region and the drain region.
This structure is very effective in reducing the cell size, since the transistor itself has a memory part. The ferroelectric material of the gate insulator film, which is now under consideration, is BaMgF4 and PbZr1xe2x88x92xTixO3.
A construction and an operation principle of this type memory cell is discussed in, for example, xe2x80x9cNonvolatile Memory FET Utilizing A Ferroelectric Material Thin Filmxe2x80x9d, Report of (Japanese) Society of Electronic Communication, CPM-78-46: 1, 1978, the disclosure of which is incorporated by reference in its entirety into this application.
In the conventional memory cell shown in FIG. 1, since the electrode underlying the ferroelectric material film has to be formed of a material such as Pt, which is hard to etch or pattern, a fine patterning is difficult. In addition, since each memory cell consists of a transistor part and a memory part, the structure is complicated. This is inconvenient to microminiaturization.
On the other hand, in the second conventional example shown in FIG. 2, since a material, such as PbZr1xe2x88x92xTixO3, having a high dielectric constant, is used as the ferroelectric material, it is difficult to form a highly reliable device. In addition, it is difficult to realize a low voltage driving, which is recently strongly demanded by users. The reason for these disadvantages will be described in the following.
In the case that a PbZr1xe2x88x92xTixO3 film is used as the gate insulating film, when the PbZr1xe2x88x92xTixO3 film is deposited directly on a silicon substrate, a natural oxide (or native oxide) layer having a thickness of about 2 nm is inevitably formed at a boundary of the silicon substrate.
A coercive electric field (applied electric field when polarization reversal starts) of the PbZr1xe2x88x92xTixO3 film is on the order of 80 kV/cm, and a dielectric constant of the PbZr1xe2x88x92xTixO3 film is on the order of 1000. On the other hand, a dielectric constant of a silicon oxide film is on the order of 4. Therefore, when the coercive electric field is applied across the PbZr1xe2x88x92xTixO3 film, an electric field as high as 20 MV/cm {=80 kV/cmxc3x97(1000/4)} is applied across the natural oxide film. However, since the natural oxide film is not an intentionally formed film, the natural oxide film is not so good in film quality, so that there is high possibility that if a high electric field as mentioned above is applied, the natural oxide film is broken down.
Here, assuming that the PbZr1xe2x88x92xTixO3 film is formed to have a thickness of 100 nm, it is necessary to apply a voltage of 0.8 V across the PbZr1xe2x88x92xTixO3 film in order to apply a necessary coercive electric field. Incidentally, in order to cause a complete polarization reversal, it is necessary to apply a voltage which is higher than 0.8 V by several ten percents. At this time, on the other hand, a voltage of 4 V (=20 MV/cmxc3x972 nm) is applied across the natural oxide film. Therefore, it is necessary to apply a voltage of 5 V or more to the gate electrode in order to cause the polarization reversal. This means that it is difficult to operate an actual device with a low voltage.
On the other hand, if the gate insulator film is formed of BaMgF4, no natural oxide film is formed since BaMgF4 does not include an oxidizing specie. However, polarizability of this material is relatively low. In addition, if the BaMgF4 film contains a crystal defect, the polarizability further lowers. Therefore, in order to constitute a satisfactory memory, it is necessary to form a BaMgF4 film having an excellent film quality, namely, less crystal defect. However, this is not so easy because of difference in lattice constant between BaMgF4 and a silicon substrate and because of other causes.
Since there exist ferroelectric materials other than oxides having a low dielectric constant, it is possible to prevent formation of the natural oxide by using the ferroelectric materials other than oxides. However, these ferroelectric materials are small in polarizability and poor in heat resistive property, and therefore, it is difficult to use these ferroelectric materials as a material used for manufacturing a semiconductor device.
Accordingly, it is an object of the present invention to provide a nonvolatile semiconductor memory which has overcome the above mentioned defects of the conventional ones.
Another object of the present invention is to provide a nonvolatile semiconductor memory configured to store information by utilizing polarization of a ferroelectric material, the nonvolatile semiconductor memory having a simple construction and a high reliability and being easy to manufacture and to microminiaturize, the nonvolatile semiconductor memory being able to be driven with a low voltage.
The above and other objects of the present invention are achieved in accordance with the present invention by a nonvolatile semiconductor memory comprising a semiconductor substrate, a gate electrode formed through a gate insulator film on a principal surface of the semiconductor substrate, a pair of source/drain regions formed in a principal surface region of the semiconductor substrate to locate the gate electrode between the pair of source/drain regions, the gate insulator film being formed of a first insulating film in contact with the principal surface of the semiconductor substrate, and a second insulating film formed on the first insulating film, the second insulating film being formed of a ferroelectric oxide having a dielectric constant of not larger than 50.
In an embodiment of the nonvolatile semiconductor memory, the second insulating film includes a material selected from a group consisting of Pb5Ge3O11, LiTaO3, YMnO3, YbMnO3, ErMnO3 and HoMnO3. The first insulating film is formed of a film selected from the group consisting of a silicon oxide film, a silicon oxynitride film and a laminated layer composed of an underlying silicon oxide film and an overlying silicon nitride film. Furthermore, a silicon oxide film or a silicon nitride film is preferably formed between the second insulating film and the gate electrode.
The inventor of the present invention discovered that if an electric field of not greater than 2 MV/cm is applied across the natural oxide film, deterioration of a semiconductor device is not facilitated. For example, if a layer of a ferroelectric material having a coercive electric field of 80 kV/cm and a dielectric constant of 50 and a natural (silicon) oxide film are stacked on each other, when the coercive electric field is applied across ferroelectric material layer, an electric field of 1 MV/cm {=80 kV/cmxc3x97(50/4)} is applied across the natural (silicon) oxide film. As mentioned hereinbefore, in order to completely polarize the ferroelectric material, it is necessary to apply an electric field larger than the coercive electric field. Therefore, even if it is assumed that polarization is caused by an electric field which is a double of the coercive electric field, the natural silicon oxide film is in no way deteriorated by this electric field.
On the other hand, in order to eliminate a defect at the time of a device manufacturing so as to realize a device operation having a high reliability, the ferroelectric material film provided as the gate insulator film is required to have a film thickness on the order of 50 nm to 200 nm. Assuming that the ferroelectric material film has the coercive electric field of 80 kV/cm, it is necessary to apply a voltage of 0.4 V to 1.6 V across the ferroelectric material film. Under this condition, on the other hand, a voltage of 0.2 V is applied across the natural oxide film. Accordingly, it is possible to start polarization by applying a voltage of 0.6 V to 1.8 V to the gate electrode. Thus, even considering that it is necessary to apply a voltage higher than that voltage by several ten percents in order to cause a complete polarization, it is possible to drive the memory with an extremely low voltage.
Furthermore, most of oxide ferroelectric materials are not deteriorated even if a semiconductor device manufacturing temperature is applied.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.